Semiconductor Random Access Memory (RAM) circuits store logic states by applying either a high voltage level (such as for logic “1”) or a low voltage level (such as for logic “0”) to the memory cell transistors that comprise the memory array. In word line voltage control circuits, the high and low (or negative) voltage levels are applied to a selected word line in a selected sector of the memory array by a decoder circuit. Current word line drivers typically used in SRAM (static RAM) devices are NAND-style decoders that are followed by one or more stages of inverters to buffer the signal. This design is generally optimal for active power and performance, but results in high power consumption due to leakage power losses. In certain operation modes, this leakage power can dominate the active power consumption of the memory chip.
FIG. 1A illustrates a logic circuit diagram of an SRAM word line driver as known in the prior art. The word line driver 100 comprises a NAND gate 102 with address inputs A and B followed by at least one inverter circuit 104. The output is the word line (WL) selected by the A and B inputs. FIG. 1B is a transistor level diagram of a word line driver of FIG. 1B. The NAND gate portion of the driver circuit 110 comprises the transistors in circuit stage 112 and the inverter section comprises the transistors in circuit stage 114. FIG. 1B illustrates a single one-bit circuit portion for a decoder that operates on an 8-bit word, thus a common A input is provided by line 113, and individual B inputs 115, denoted Bn (e.g., b1-b7) are provided to the individual NAND circuits. In the case of eight B inputs, the b0-b7 inputs would represent half of the partially decoded word lines. For a 64-word line circuit, there would be eight different A inputs and eight different B inputs. To select word line 0 (WL0=1), the decoder would select A0B0, and to select word line 63, the decoder would select A7B7.
A typical SRAM circuit may have 128 or more word lines. During normal operation, only one word line is in use and the remaining lines are inactive. FIG. 1C illustrates a word line driver comprising a plurality of NAND gate driver circuits of FIG. 1B. FIG. 1C illustrates a single eight-bit portion of a decoder circuit for a single A input and eight B (b0-b7) inputs. For the circuit of FIG. 1C, only one word line output is active when a valid address is selected for the memory block controlled by decoder circuit 120. The active word line (WLnl ) is consuming active power, and the remaining inactive word lines are all consuming standby power. In a typical NAND gate based decoder circuit, the standby power may be three times the active power. This is significant power consumption when one word line is selected, however, in the case when the memory is not active every clock cycle, the proportion of standby power consumed to active power becomes even more significant.
The source of such high leakage power is described in relation to the transistor circuit illustrated in FIG. 1B. In such a circuit, generally it is the inverter stage 114 that causes a majority of the standby power consumption. The inverter stage 114 consists of a PMOS (p-channel metal-oxide semiconductor) transistor 116 and an NMOS (n-channel metal-oxide semiconductor) transistor 118. In order to maintain a certain performance characteristic, the PMOS transistor 116 is typically a relatively large transistor to provide adequate output power characteristics.
When a word line for a particular NAND circuit is selected, the word line (WL) output 120 is logic high. In this case, the gate of PMOS transistor 116 is at logic 0 (ground) and the gate of NMOS transistor 118 is at ground. The leakage current of the inverter circuit consists of leakage through the NMOS transistor 118, IL-NMOS, and this is relatively minimal at least in part because the NMOS transistor 118 is smaller than the PMOS transistor 116. The worst case leakage current scenario occurs when the word line for the NAND circuit is deselected, and the WL output is logic 0. In this case, the gate of PMOS transistor 116 is at logic 1 (Vdd) and the gate of NMOS transistor 118 is at logic 1 (Vdd). The leakage current in this case consists of the leakage through the PMOS transistor 116, IL-NMOS, which is generally significantly larger than IL-NMOS With respect to the entire word line driver circuit, the critical path for the active state is when the NAND stage transistors 112 are pulled down, and the PMOS transistor 116 of the inverter circuit is pulled up. This results in the maximum leakage current through the word line driver.
The worst case leakage current scenario is when either none or only one word line is selected. In this case, a maximum amount of leakage power is consumed thus accounting for the significant consumption of standby power for NAND-gate based word line drivers.